FIG. 4 is a diagram showing a single stage amplifier circuit as an example of a prior art microwave integrated circuit. In FIG. 4, reference numeral 1 designates a field effect transistor(FET). An input matching circuit 4 is constituted by microwave lines 42, 43, and 44, capacitors 41, 46, and 47, and a resistor 45 and is connected between the gate of the FET 1 and an RF(radio frequency) signal input terminal 2. The capacitor 41 and the microwave lines 42 and 43 are connected in series and are provided between the RF signal input terminal 9, and the gate of the FET 1. A node between the microwave lines 42 and 43 is connected with a gate bias terminal 21 via the microwave line 44 and the resistor 45. The ends of the resistor 45 are grounded via the capacitors 46 and 47, respectively. An output matching circuit 5 is constituted by microwave lines 51, 52, and 54, and capacitors 53 and 55 and is connected between the drain of the FET 1 and an RF signal output terminal 3. The capacitor 53 and the microwave lines 51 and 52 are connected in series and are provided between the drain of the FET 1 and the RF signal output terminal 3. A node between the microwave lines 51 and 52 is grounded via the microwave line 54 and the capacitor 55. A node between the microwave line 54 and the capacitor 55 is connected to a drain bias terminal 8. The gate bias terminal 21 is connected to a ground terminal 7 via an external gate bias voltage power supply 22 while the drain bias terminal 8 is connected to the ground terminal 7 via an external drain bias voltage power supply 10.
Next, a description will be given of the operation.
When a proper gate bias voltage is applied from the external gate bias voltage power supply 22, the amplifier circuit is driven by a drain current flowing from the external drain bias voltage power supply 10 to the FET 1 via the drain bias terminal 8 and the microwave lines 54 and 51. An RF input signal supplied to the RF signal input terminal 2 in this operating state is amplified by the operation of source grounded FET 1 and is output from the RF signal output terminal 3. The resistor 45 and the capacitors 46 and 47 constitute a low-pass filter for preventing the FET 1 from oscillating. The resistor 45 also functions to prevent a surge voltage breakdown of the gate of the FET 1.
In the prior art microwave integrated circuit constructed as described above, the drain current varies depending on variations in the pinch off voltage of the FET 1. Therefore, it is necessary to adjust the gate bias voltage and if it is not adjusted, such as 1 dB compression point output P.sub.1dB, power gain, voltage standing wave ratio (VSWR), and power consumption vary.
In addition, a short stub is used as a microwave line to reduce the chip size and a capacitor having large capacitance is used as the capacitor 47 in a monolithic microwave integrated circuit (hereinafter referred to as "MMIC"). For this reason, a large capacitance exists at the gate bias terminal 21. Therefore, the switching response of FET 1 is delayed when the FET 1 is driven on or off by a pulse signal such as a radar signal.